When the mites go up…

It doesn’t seem obvious really. Going underground into caves, removing stalagmites and analysing their isotopic composition isn’t the first thing you would do to look for past climate information. But for nearly 40 years, there has been an active, and growing research community that investigates the climate records preserved in these archives. Stalagmites have recently received high profile use in climate reconstructions, for example records from China and Norway have featured in Moberg’s last millennium temperature reconstruction; in a northern hemisphere temperature reconstruction of the last 500 years and even been debated here on RealClimate. So it seems timely to review why on (or even under) earth should research go underground to look at surface climate.

To do that, we need briefly to explain how stalagmites are formed. Most simplistically, to grow a stalagmite you need water, and that water has to be saturated with carbon dioxide. Then this water drips from a cave roof, the carbon dioxide in the water will ‘degas’ into the atmosphere, and as part of that process calcium carbonate will form, which will form a stalagmite. Both the presence of water, and the fact that the water is saturated with carbon dioxide, can provide information about the surface climate. The water was, at one time in the past, surface rain or snow, and should contain information about that rain or snow through the composition of its isotopes. And the carbon dioxide saturation comes, not from the atmosphere, but the soil above the cave. Soil carbon dioxide concentrations are orders of magnitude greater than atmospheric, and there is a complex relationship between the concentration of soil carbon dioxide in cave drip waters, temperature and soil moisture.

Thus there could be some climate signal preserved within stalagmites, the question is how to decode it. The surface rain will interact with the surface soil and vegetation, which may alter any climate signal containing in the rainwater, or create new soil derived signals, or probably a mixture of the two. After a period of time that will depend on climate, seasonality, vegetation, etc. the water will reach the ground water. In the ground water, it will probably mix with waters of differing ages, smoothing any climate signal it contains. The nature of ground water flow may also introduce non-linearities into the signal. The simplest example is the overflowing bath scenario – imagine that filling your bath up at an increasing rate represents increasing rainfall, and that the bath is your groundwater store, and the plughole and the overflow are the outlets feeding stalagmites. The plughole stalagmite will respond first to the rainfall, the overflow stalagmite will be delayed untill the bath is full. As the bath fills and the storage time increases, the plughole stalagmite will preserve an increasingly smoothed water signal.

Therefore, it is a complex system. Over the last 40 years of stalagmite palaeoclimatology, the bulk of the research community was interested in the timescales of ice ages, as one of the big advantages of working with stalagmites is that they can be dated by the natural decay of uranium and thorium isotopes back to around 500,000 years. Over those timescales and temperature changes of 10°C or more, stalagmites are pretty convincing at recording the 1st order climate changes (e.g. glacial-interglacial changes, Dansgaard-Oeschger events, etc..) as the climate signal is much greater than the noise induced in the soil/vegetation-groundwater-cave system, and several papers a year can be read in journals such as Science and Nature (in particular, the Hulu cave record was truly exceptional).

Over the last 10 years or so, with increased interest in climate change and climate variability over the last millennium, researchers have started to use stalagmites to look at the more recent past and at higher time resolution. This is despite the climate signal being much smaller (e.g. changes in temperature of less than 1 °C over the last 1000 years) yet the same level of noise in the soil/vegetation-groundwater-cave system. So this is a much harder task, requiring careful sample and site selection. Given the complexity of the signal transfer from the surface to the cave, the only approach is to: (1) work with stalagmites that were actively growing when sampled (so we know the precise age of the top of the archive, and that they were deposited over the period of instrumental rainfall and temperature data), (2) that we use stalagmites which have annual growth rings (in the same manner as tree rings, many stalagmites have annual rings too) so that a precise chronology can be obtained, and then (3) to analyse whichever proxy we are interested in at highest resolution possible to be able to calibrate that proxy against instrumental climate records over the period they exist. If that methodology sounds familiar, then it is because it is very similar to that used with other proxy climate records such as tree rings. The figure (from Proctor et al, 2000) shows an example of that kind of calibration – in that case against regional sea surface temperature records.

Some caveats are now needed. Firstly, all three points listed above have to be fulfilled – any two of the three just won’t give the precision in terms of chronology or climate calibration necessary to inform the past climate variability debate (a problem with the Norwegian record that featured in Moberg et al is that it failed all three tests). Secondly, depending on the local site, some stalagmites just won’t record high frequency (e.g. annual) climate variability – that signal could be lost if waters are mixed in with a large reservoir of ground water. But they may be excellent at recording the low frequency (decadal to centennial) climate signal, something that proxies such as tree rings have more of a struggle to do. Thirdly, it is quite possible that stalagmites will have a seasonally biased climate signal. For example, if the cave climate varies seasonally such that stalagmite deposition only occurs for part of the year, or if the rainfall only comes during the wet season, then it can only record that season’s conditions.

Stalagmites are sure to feature in forthcoming reconstructions of climate over the last millennium, as the best samples can form continuously for thousands of years, sometimes with continuous annual growth rings, and with little or no growth related trends, all of which are good conditions for preserving low frequency climate variability. But the complexity of the climate transfer function means that the three tests of chronology and calibration need to be fulfilled for any stalagmite record to be demonstrated to be both accurate and precise enough to be used in a proxy climate reconstruction of the last 1000 years.

30 Responses to “When the mites go up…”

What is the reasoning behind using stalagmites vs. stalactites? At first glance it would seem that a stalactite would lend for less error since it does not (as easily) come in contact with ground water.

Great article. It’s fascinating to see the different ways information about the past can be obtained. That said, I have some questions…
How do you deal with variables like changes in cave geometry? It seems to me that as time goes on, the small cracks that water seeps through can get blocked or a chunk can fall, drastically changing the situation. Even gradual changes could alter the growth pattern over time…
Obviously, this is the purpose of the calibration step and the annual growth rings, but it still seems that any results will consist of numerous assumptions about the history of the stalagmite and its environment.
So the question is how do you tell the difference between the signal and the noise in these measurements?

Some caves are closed; some have openings to the outside air. A large cave with a small opening will ‘breathe’ as the outside air pressure changes. If the opening is sometimes blocked by high water the breathing will stop and start at that point in time. If a collapse opens or closes an opening, breathing stops. This just from the point of view of a sometime caver (non-cavers call it spelunking). I don’t know how or if that changes deposition — but gathering several samples from different caves or different locations in a cave, or knowing that an area in a cave had to be dug open to get into it the first time while another room nearby had been open to outside air, would allow comparisons.

Some replies from the guest commentator:
1. Absolutely correct – there is less potential for interactions with the cave atmosphere if a stalactite is analysed. But, stalactites typically grow, at first, as an ever lengthening straw. These are fragile, and will break and fall with any seismic movements, or animal or human contact. So they rarely grow for more than a century or two, which limits their use for looking at the longer term, low frequency climate variations. And, if they do survive, they normally ‘clog up’, either with sediment or carbonate precipitate, and the drip water typically find another route bypassing the blockage. This leads to a complex stratigraphy, and in general, stalagmite stratigraphy is simpler.

3. Again, yes, absolutely, this could be a problem. The good news is that cave evolution is typically over longer timescales than of interest here, so one would be very unlucky to see such an effect over, say, the last 1000 years. My rule of thumb is that, if I see a sudden (non-linear) change in a (what I think would normally be) a climate signal in a stalagmite, I would worry that something such as a blockage of the cave entrance, or a new entrance opening, has occurred. Hank Roberts (comment 5) provides an excellent test through the simple use of adequate replication. Another test would be to use a ‘multi parameter’ approach – that is to use one parameter that might be sensitive to changes in cave air circulation, and another that should be sensitive to surface climate.

4. The width of growth rings varies with cave air temperature, which is, in deep caves, pretty constant as commented. But it sensitive to other factors, particularly the amount of CO2 in the dripwater, which is related to soil temperature and moisture, as well as the drip rate, and the cave air CO2 concentration. If you can assume that the cave air CO2 is pretty constant, then stalagmite ring width is most sensitive to the dripwater CO2 concentration when dripping occurs all year long, and drip rate becomes more important at extremes of drip rate. This is a crude summary of the work of Prof Wolfgang Dreybrodt at Bremen if anyone likes to follow this up.

I am quite surprised that in the Proctor graph there is a high correlation between the SST North of Iceland (a mix of polar waters from the East Greenland current and the warmer Irminger current coming from the South) and the growth rate of stalagmites in Scotland!
Is there any explanation for this connection?

Even more interesting is the correlation between the Asian monsoon and the temperature changes in the North Atlantic. Seems that the same origin is at work (insolation changes/solar cycles?)

Andy, I’m aware of a paleo person here in California who has very high expectations for a just-harvested stalagmite that is noted to have stopped growing about 5,000 years ago. From your discussion it seems that the data would have no value absent an ability to calibrate to the instrumental record, and yet this person obviously believes otherwise. Could you explain? You may already know who I’m talking about, but I’ll email you the link just to be sure.

Some more replies from the guest commentator:
Comment 6: An interesting question. My gut opinion is that most caves would be useful. The deeper underground one goes, the greater the smoothing of the surface climate signal, due to mixing of this signal with older groundwaters. So deep caves are more likely to be better for recording low frequency climate changes. Near-surface caves would, in contrast, be better at recording high frequency climate changes. Both those statements presume a climate signal being transferred from the surface via the groundwater. A stalagmite that is recording a climate signal via the cave atmosphere instead might disobey this rule of thumb, if, for example, a deep cave is stongly ventilated and therefore connected to the surface.

Comment 9: yes, over glacial-interglacial cycles, stalagmites are very good at showing these first order climate teleconnection. With respect to the first question relating to the figure, the co-relation with SST is part of the wider N Atlantic correlation between SST, winter NAO and precipitation in the region. NW Scotland has mean annual rainfall that strongly correlates with the winter NAO; wetter conditions at the cave site saturates the overlying soil and limits soil CO2 production, limestone dissolution and therefore stalagmite growth rate, at least over this time period.

Comment 10: I think it is much harder to be confident of the climate signal in such a case. Two options are: (1) replicate the signal. If it replicates in several stalagmites then it is more likely to be a regional climate signal rather than a local site specific effect. Or: (2) use a multi-parameter approach, as I mentioned in the earlier post. If all parameters (isotopes, lamina width, etc…) show the same climate signal, then one would have more confidence that it is real.

An interesting article. Please point us to some work that has shown good results in terms of showing historical climate trends (if it exists yet). (We can’t learn much from the attached graph. Advice to other readers: to read better, you need to copy and paste into another package). Of most interest would be work that has cocentrated on stalagmites as a proxy. As soon as you start to mix proxies, greater uncertainty creeps in with regard to the effectiveness of any specific method.

Your criteria (1) is interesting in that it requires a continuous proxy from the past right up to at least the early parts of the instrumental records. How much is this required or met for other proxies, such as individual tree rings for temperature, or ice cores for CO2 concentrations? (others can perhaps answer this if not you).

I’m surprised at the questions asking how you deal with ‘3rd party’ variables. Such uncertainties are valid for pretty well any proxy method, so nothing new. There are probably far fewer variables for a stalagmite in a cave than for a tree exposed to the elements.

Re: #12
Try this: Frisia et al., 2003, Earth and Planetary Science Letters, 216:411-424. I’m not saying it’s a “good” article, it’s just the one I know. If you don’t have a subscription, I can send you a pdf.
On the formation of laminae, try Frisia et al., 2000, Journal of Sedimentary Research, 70:1183-1196.http://www.ncdc.noaa.gov/paleo/speleothem.html is a public database of stalagmite data.

Replying to comment (12): The Smith et al (2006) paper listed at the end of the piece is one where only stalagmite series are compared. Three northern hemisphere annual ring width series in stalagmites are analysed, one from NW Scotland (the same as in the figure), one from Italy and one from Beijing (all three series are published and data is available at the World Data Centre). Although each stalagmite of course responds to local climate factors, the low frequency signal in each sample is similar and correlates strongly with N Hemisphere temperature.

Comment (15): sure! Historically, research has been biased towards Europe, USA and Canada e.g. where the early researchers were located. But in the last decades research is truely global, and so are the research sites. I would guess that under-researched regions are those where access is more difficult, for practical or political reasons.

The earth has been in a cycle of cooling and warming as long as it has existed. We are in a peak of one of the warming cycles now. Scientist acknowledge the earth goes through cycles, but what I find interesting is the end of the long cycle according to the myan’s is in 2012. […edited due to apparent spam attempt, see comment #23]

My understanding of this is that stalagmites can (with precautions) serve as another proxy for climate, and thereby make the total set of indicators & proxies more robust (and maybe the skeptics will finally accept the hockey stick — though I won’t hold my breath).

Is there anything new stalagmites might tell us, in addition to what the other proxies are telling?

I just started teaching methods/stats from criminal justice, and we were just talking about reliability, validity, precision, accuracy…..and proxies.

That appears to be this year’s search string target for some search engine spamming competition, near as I can tell. It’s being posted in all sorts of websites, in hopes back-links make it appear legitimate and it gets listed high up in searches at Google et al.

[Response: Thanks for the heads up Hank. We’ve eliminated the offending link from comment #20, and flagged it for future reference. -mike]

#20, yes, I also heard that about the Mayan calendar….sort of like an apocalypse thing (BTW the ancient Mayan calendar is much more accurate than our calendar — in that it runs in 52 year cycles). And then there are the Armageddon Christians talking fiercely about world’s end.

If these religions are intuitively onto something that has any scientific possibilities….it must be runaway warming (the limited one, like the ones that happened 55 & 251 mya, but that do enormous damage to life on earth). I wonder if the stalagmites are telling us any secrets like this :)

So, perhaps 2012 is the year we reach the runaway tipping point of no return…or the year the scientists finally are able to tell us with scientific certainty that we have already passed that point back in 2009.

Anyway, I’d suggest reducing our GHGs way down & proving all these guys wrong! And proving the contrarians right — no GW.

#21: I think the main strength of stalagmites, at least when looking at the climate of the last 1000 years or so, is that stalagmites typically grow for hundreds, thouands or rarely tens of thousands of years. So an individual sample is likely to have grown for much or all of the last 1000 years, and with little or no growth related trends. Other proxies, trees for example, have shorter growth periods and have growth related trends that have to be dealt with. So, in the specific point of the ‘hockey stick’, a few more stalagmites in such a reconstruction should help us even further constrain the low frequency climate variations that have occured over the last 1000 years.

The other advantage I would say is that they can contain information on both temperature and rainfall. Many proxies record temperature, but far fewer provide rainfall. Careful stalagmite selection can yield a strong rainfall signal e.g. by chosing a sample that only drips seasonally, and therefore whose annual growth rate may give a signal that is dominated by the number of months in a year that the water was dripping.

“… three Apollo missions (15, 16, and 17) returned deep-drill core samples from three different sites on the Moon. The cores drilled more than 2 meters into the lunar regolith (the layer of broken rock and dust covering the Moon).

“The deepest samples brought up by those drill cores were 2 billion years old, and largely unchanged since they were laid down,” Spudis says. And what a surprise recent re-analysis has revealed. “The lunar regolith traps particles from the solar wind. And drill cores show that the solar wind had a different chemical composition 2 billion years ago than it does today. There’s no known explanation for that in solar theory. But that discovery is crucial for understanding the formation of Earthâ��and also the evolution of stars.”

As an old (now inactive)caver and leader of a research program (1990-1995, see here ) on underground climatology I would like to add that airflow pattern in a cave (which, even if air flow is very low, modulate cave air CO2 concentration) can easily be detected by monitoring radon in cave air: many caves have a seasonal breathing pattern (in and out) and often short-term airflow oscillations which might have an influence on the stalagmite formation. And indeed, CO2 concentration in caves is highest in fissures having a contact to top soil, where microbial activity (highest during warm periods) is the main producer.

Al Gore’s movie has had one positive effect. It has people talking. What we need the most now, if those same people making a commitment to acting. I am trying to do actual work that may have just as big an impact and could use some help. Check http://www.brewsmith.com and see if you would be interested in doing more then discussing solutions! Thanks

Re: #10. The case for the study mentioned follows just the same options that Andy suggested. The recently collected stalagmite referred to is part of a large study involving stalagmites from more than one cave in a given region and will examine several proxies in each stalagmite. Thus, any climate signal will be checked for its reproducibility in order to determine whether or not it’s regional or local in extent. Additionally, as mentioned in the original post, inactive stalagmites can provide an interesting look at low-frequency climate variability. Finally, we’re just guessing that the stalagmite stopped growing ~5,000 years ago. We’re crossing our fingers, actually, that it is much more recent than that.

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